Central Nervous System (CNS)

NERVOUS TISSUE

The nervous system consists of all nervous tissue in the body. It is divided anatomically into the central nervous system and the peripheral nervous system.

Central Nervous System (CNS)

The CNS consists of the brain (encephalon), which is enclosed in the skull, and the spinal cord, which is contained within the vertebral canal. Nervous tissue of the CNS does not contain connective tissue other than that in the three meninges (dura mater, arachnoid membrane and pia mater) and in the walls of large blood vessels. Collagenous fibers or fibrocytes/blasts are consequently not observed, which is quite unlike other tissues. Because of the absence of connective tissue, fresh CNS tissue has a very soft, somewhat jelly-like consistency. The two major classes of cells that make up the nervous tissue are nerve cells, neurones, and supporting cells, glia.

Neurons

The vast majority of neurones is generated before birth. Persisting stem cells give rise to a small number of new neurones throughout the lifetime of mammals, including humans. The permanent addition of neurones may be important for the maintenance and plasticity of some parts of the CNS, but it is insufficient to replace neurones that die because of disease or acute damage to the CNS. Neurones should last a lifetime. Mature neurones are not mitotically active, i.e. they do not divide.
Neurones are generally large cells. Neural activity and its control require the expression of many genes, which is reflected in the large and light nuclei of most neurones. The keys to the understanding of the function of a neurone lies in (1) the shape of the neurone and, in particular, its processes, (2) the chemicals the neurone uses to communicate with other neurones (neurotransmitters) and (3) the ways in which the neurone may react to the neurotransmitters released by other neurones.

The shape of the neurone and its processes  Neurones have long processes, which extend from the part of the cell body around the nucleus, the perikaryon or soma. The processes can be divided into two functionally and morphologically different groups, dendrites and axons. Dendrites are part of the receptive surface of the neurone. As a rule, neurones have one to several primary dendrites, which emerge from the perikaryon. Primary dendrites may divide into secondary, tertiary etc. dendrites. Dendrites can be smooth, or they can be studded with small, mushroom-shaped appendages, which are called spines. Each neurone has as a rule one axon, and never more than one axon which emerges from the perikaryon or close to the trunks of one of the primary dendrites. The point of origin of the axon from the perikaryon is the axon hillock. The axon may, like the dendrites, branch as it travels through the nervous tissue to its destination(s). The axon is the "transmitting" process of the neurone.

The axon forms small, bulb-shaped swellings called boutons at the ends (terminal boutons) or along the course (boutons en passant) of its branches. Synapses are morphologically specialised contacts between a bouton formed by one neurone, the presynaptic neurone, and the cell surface of another neurone, the postsynaptic neurone. Synaptic vesicles contain the neurotransmitters. Synaptic vesicles typically accumulate close to the site of contact between the bouton and the postsynaptic neurone. The release of the neurotransmitter from the synaptic vesicles into the synaptic cleft, i.e. the space between the bouton and the postsynaptic neurone, mediates the transfer of information from the pre- to the postsynaptic neurone. Both the release of the synaptic vesiscles and the mediation of the response to the transmitter require membrane-associated specialisations - the pre- and postsynaptic densities.

The shape and orientation of the dendritic tree of the neurone determines the amount and type of information that may reach the neurone. The course of its axon determines to which neurones this information may be passed on. The location of the neurone within the CNS determines to which major system the neurone belongs.

There are several hundred functionally different areas, i.e. groups of neurones, in the CNS. Based on their location, the shape of their dendritic tree and the course of their axon, several thousand types of neurons can be distinguished in the CNS.

Transmitters

Neurotransmitters either excite or inhibit the postsynaptic neurone. The most prominent excitatory transmitter in the CNS is L-glutamate. The most prominent inhibitory transmitter in the CNS is GABA(gamma-amino butyric acid). Other "main" neurotransmitters are e.g. dopamine, serotonin, acetylcholine, noradrenaline and glycine. Each neurone uses only one of the main transmitters, and this transmitter is used at all synaptic boutons that originate from the neurone.
One or more of the "minor" transmitters (there are several dozens of them - such as cholecystokinin, endogenous opioids, somatostatin, substance P) may be used together with a main transmitter.
The molecular machinery that is needed to mediate the events occurring at excitatory synapses differs from that at inhibitory synapses. Differences in the morphological appearances of the synapses accompany the functional differences. The pre-and postsynaptic densities are typically of equal width, or symmetric, at inhibitory synapses. The postsynaptic density is thicker than the presynaptic density at asymmetric synapses, which are typically excitatory.

Receptors

Usually there exists a multitude of receptors which are all sensitive to one particular neurotransmitter. Different receptors have different response properties, i.e. they allow the flux of different ions over the plasma membrane of the neurone or they may address different second messenger systems in the postsynaptic neurones. The precise reaction of the neurone to the various neurotransmitters released onto its plasma membrane at the synapses is determined by the types of receptors expressed by the neurone.

Suitable Slides

sections of spinal cord - H&E, luxol fast blue/cresyl violet (LFB/CV), toluidine blue, Giemsa

Thoracic Spinal Cord, sheep - LFB/CV
Most neurones have a light, large nucleus with a distinct nucleolus. The cytoplasm of many neurones contains fairly large amounts of rough endoplasmatic reticulum, which may aggregate within the cytoplasm of the neurone to form Nissl-bodies. Nissl-bodies are prominent in motor neurones located in the ventral horn of the grey matter of the spinal cord. The neurites are difficult to identify in most types of stained sections. Only the most proximal segments of the primary dendrites are seen clearly. The size of the perikaryon depends on the level of activity of the neurone and the length of the processes which the neurone has to support. An usable range for the size of the perikaryon would be 15 - 50 µm, although much smaller and much larger neuronal perikarya exist.
Draw the spinal cord at low magnification and indicate the distribution of grey matter and white matter. Find a nice group of neurones in the grey matter and draw them at a high magnification. Finally, have a look at the white matter and identify the nuclei of glial cells. You will find similar nuclei also in the grey matter.
Thoracic spinal Cord - H&E, silver stain
These slides show the same major features as the LFB/CV stained sections. Try to identify neurones (primary dendrites, Nissl-bodies) and glial cell nuclei in the H&E stained section. Differentiate between grey and white matter. The LFB stain showed the myelin sheath nicely. In the H&E stained section we instead can see large, cross-sectioned axons in the white matter. The feltwork of nerve fibres, neuronal and glial cell processes is also called neuropil.
Part of the cytoskeleton of neurones is (like the reticular connective tissue fibers) argyrophilic, i.e. they "love" silver and can be stained by silver stains. Aside from the neurones and their processes, fine fibrils are visible in the neuropil. Many of the fibrils represent axons travelling in the grey and white matter of the spinal cord.